This invention relates to hydrostatic transmissions as well hydrostatic transaxles which are now in increasing usage for lawn care and other outdoor power equipment duties as the preferred choice for power transmission drive lines; for example, lawn and garden tractors; pedestrian walk-behind mowers and snow blowers.
Hydrostatic transaxles of the type currently sold in the marketplace require careful assembly and service practices in order to avoid certain problems occurring that may result in lower than expected operational life of the product. Further, the application in which the device is used must not overload the transmission to such a extent that the rise in the temperature of the operating fluid lowers performance, most noticeable as a drop in vehicle speed. Hydrostatic transmissions operate most effectively and efficiently when they are constructed with exceedingly small clearances between their reciprocating and sliding elements. The transmission of power by such hydrostatic transmissions has now become widespread and when well designed for the intended application, the attendant small fluid leakage loss from the internal pressurised circuit during operation which is inherent with this type of speed changing device is generally considered unimportant as the resulting retardation in vehicle speed most often goes largely unnoticed by the end user. Hydrostatic transmissions work well and have a long and useful life so long as the level of contamination suspended in the power transmission fluid remains low. High levels of contamination carried by the power transmitting fluid can rapidly wear out the aforementioned fine clearances resulting in an increase in fluid leakage, especially during high pressure operation. However, wear can take place even when the level of contamination in the hydraulic fluid remains low, and such wear accelerates when the temperature of the working fluid becomes so high that the fluid viscosity decreases to the point when lubrication over the sliding surfaces is inadequate. In this regard, it is preferable although not essential, to keep the lubricating fluid in the gear compartment segregated from the fluid in the hydrostatic compartment.
It is important during initial product assembly in the factory as well as at subsequent service repair intervals to attempt to minimise the chance for contamination to enter the chamber surrounding the working components of the hydrostatic transmission. The contaminated of such components can occur during handling on the assembly lines, especially if the mechanical gearing is assembed at the same time. It follows therefore, that if the hydrostatic transmission could be fully assembled with the minimum of gear components being handled, and sealed in the housing prior to the remaining elements associated with the reduction gearing and differential being added, there would be advantage.
It is also of great importance to include such features in the design of the device to help ensure that the temperature of the power transmitting fluid remains within acceptable limits. At present with the hydrostatic transaxles presently manufactured, the outer housing provides a sump into which a center section is located and where such fluid passages are contained within the center section. As a result, effective cooling of the fluid passing through the passages is hindered as the fluid surrounding the center section acts as a insulating medium to slow down the rate of heat transfer from the power transmitted fluid in the passages to the surrounding housing radiator. It would therefore be an advantage if the fluid passages connecting the pump and motor of the hydrostatic transmission could be positioned integral with the wall structure of the surrounding housing and as close as possible to the input shaft driven cooling fan.
In the present art of hydrostatic transmissions when used in combination with related transaxle apparatus, it is preferable that the parking brake device be included internally inside the transaxle housing rather than being situated on the exterior to be affected by exposure to damp and dirty conditions in the environment. However, location of such a brake internally within the transaxle housing is a problem if worn brake lining debris finds its way into the power transmitting fluid of the hydrostatic transmission. What is therefore needed is a protected parking brake for the hydrostatic transaxle for location inside the same chamber where the gearing of the transaxle is located such that debris worn away from the brake is prevented from entering the hydrostatic compartment. What is further needed is an internal brake and disengage mechanism for a transaxle apparatus whereby any contamination generated by these two mechanisms does not find its way into the hydrostatic compartment, but will still allow simple repairs to be effected. For example, the replacement of a worn brake shoe without having to dissemble the housing containing the hydrostatic transmission. In many hydrostatic transaxles presently manufactured, the hydrostatic transmission as well as the gearing and differential all operate in the same fluid bath, this being often referred to as a common sump design. A disadvantage of the common sump approach is that once the fluid is heavily contaminated with water forming a sludge mixture with material worn from the gears, it shows up as a noticeable drop in performance, and the unit is either a throw-away or requires a complicated and expensive repair. There would therefore be an enormous advantage if the compartment containing the gearing could be easily assessable to be cleaned and on occasion, receive clean lubricating fluid. There would be a further advantage if the gearing were to be operating with the bare minimum of lubricating fluid rather then be fully submerged as is usual in the common sump arrangement. A low level of fluid for the gearing corresponds to an improved overall operating efficiency of the hydrostatic transaxle as the turning losses are less.
From one aspect the invention consists of a housing structure for a hydrostatic transaxle where the housing construction comprising three housing elements that inter-relate to form a chamber for the hydrostatic transmission components and a chamber for the geared components. An input shaft is supported in the housing and extends into the chamber containing the hydrostatic transmission to drive the hydraulic pump, and where an output shaft is also supported in the housing to extend into that chamber containing the geared components. In instances when a mechanical differential is also located within the chamber containing the geared components, the output shaft then comprises two shafts that extend from the differential in opposite directions. Within the chamber containing the geared components, the output shaft or shafts is drivingly engaged to the speed reduction gears and where the gears are driven by a connecting shaft that forms the power transmitting link between the hydraulic motor in the hydrostatic chamber and the geared components in the gear chamber. A bridging element spans across the chambers, the bridging element can be in the form of a cylindrical bearing member seated in a pocket in the larger of the three housing elements as well as in respective pockets provided in the two smaller housing elements. Anaerobic sealant being applied to all three pockets during assembly of the device in order to prevent any loss of fluid from the chambers, and where a rotary shaft seal or filter element that act as a fluid barrier can be placed at the interface between the housing elements to prevent at a minimum, substantial sized particles of contamination generated in the gear compartment from entering the hydrostatic compartment. Although best achieved through the inclusion of a fluid barrier such as a rotary seal surrounding the shaft connecting the hydrostatic unit to the gearing allowing a substantially dry sump for the gears to operate in, the alternative being a filter element such as a sintered hollow plug disposed in a communication passage connecting the compartments together to provide a solution for a common sump.
It is therefore an object of this invention to at least minimise and preferably entirely eliminate the danger of contamination generated in the gearing compartment from entering the compartment containing the hydrostatic transmission.
It is a further object of the invention to situate the parking brake inside rather than outside the transaxle. Presently most parking brakes for transaxles are located external of the housing on a brake-shaft such that the brake assembly is accessible should worn brake shoes need replacing. However, in the event that the vehicle operator forgets to release the brake before driving the vehicle, the resulting heat generated at the brake can be high enough to cause the seal surrounding the brake-shaft to fail. A failed oil seal results in the loss to the environment of the lubrication fluid in the sump and most often the hydrostatic unit needs to be replaced. It is therefore an object of the invention to position the brake assembly within the chamber containing the gearing so that any heat generated at the brake is absorbed by the surrounding lubricating fluid, or in the event of a failed oil seal, a repair can be undertaken with ease. It is a still further object of the invention to provide a new solution that would allow worn elements in the gear compartment such as the brake pads of an internal parking brake to be replaced without having to disturb the hydrostatic components. The ease of replacing worn brake parts without the dismantling of the whole transaxle housing structure would be economically advantageous to the industry thereby allowing the service agent to effect such simple repairs in the field.
Operational performance of the hydrostatic transmission can be enhanced by improving the heat dissipating characteristics of the surrounding housing structure. It is therefore a still further object of the invention to improve the performance of the hydrostatic transmission and make its less sensitive to wear from contamination by positioning the fluid passages interconnecting the hydraulic pump and hydraulic motor integral with the wall structure of the housing and as close as possible to the input shaft driven cooling fan.
In one form thereof, the hydrostatic transaxle of the invention comprises an axle driving assembly comprising a housing constituted by first and second housing elements disposed adjacent one another on one side of a parting plane, and a third housing element disposed on the opposite side of said parting plane and connected to each of said first and second housing elements, said first and third housing elements together defining a first chamber in which a hydrostatic transmission is disposed and said hydrostatic transmission comprising a hydraulic pump and hydraulic motor, and said second and third housing elements together defining a second chamber in which a mechanical differential is disposed, said hydrostatic transmission and said mechanical differential being operatively connected together by speed reducing gearing disposed within said housing; and wherein one of said housing elements is provided with first and second fluid passages integral with its wall structure for fluidly coupling said hydraulic pump with said hydraulic motor.